Single mode optical fiber with particular graded index as well as optical communication system

ABSTRACT

The present invention relates to a single mode optical fibre comprising a first central region having a radius r 1 , a maximum refractive index value n 1  and at least one second ring surrounding said first central region, which second ring has a radius r 2  and a minimum refractive index value n 2 , wherein n 2 &lt;n 1 . The present invention furthermore relates to an optical communication system for multi-channel signal transmission.

The present invention relates to a single mode optical fibre comprisinga first central region having a radius r1, a maximum refractive indexvalue n1 and at least one second ring surrounding said first centralregion, which second ring has a radius r2 and a minimum refractive indexvalue n2, wherein n2<n1.

Such a single mode optical fibre is known per se from U.S. Pat. No.5,905,838, wherein in particular FIG. 4 schematically shows thenormalised refractive index difference, as a function of radial positionfor the four region fibre. Thus the silica core doped with germanium issurrounded by an annular region of depressed index, in this instance,composed of fluorine-doped silica. Surrounding the region is agermano-silica ring, in turn within an outer cladding region of, forexample, undoped silica. The core region shows a depressed-index dipcharacteristic of MCVD-produced fibre. Such a fibre is also defined as aso-called “double window WDM (“wavelength division multiplexed”) fibre”,which fibre is used in so-called metro networks or long-distancenetworks. Such networks are characterized by medium-length transmissiondistances of up to a few hundred kilometres and a large number of nodes,where branches and/or connections to other (parts of) networks arepresent. The optical fibres via which the transmission of signals takesplace in such networks are preferably suitable for high transmissionrates at a large number of different wavelengths.

The article “Maximum effective area for non-zero dispersion-shiftedfibre” discloses refractive index profiles in which a fibre of this typehas a dispersion slope of 0.08 ps/nm².km at 1550 nm. The effective areaat 1550 nm is in the 45–110 μm² range. Further details with regard tothe slope for obtaining a satisfactory equilibrium of characteristics,in particular as regards the dispersion slope, macro bending andeffective area, are not known therefrom.

U.S. Pat. No. 6,396,987 relates to an optical fibre for use in WDMtransmission systems, wherein the effective area is larger than or equalto 60 μm² and the dispersion value ranges from 6 to 10 ps/(nm.km).

European patent application No. 0 249 230 relates to a method formanufacturing a preform. Since said document only relates to preforms,no specific details are provided with regard to the optical fibre drawnfrom such a preform, such as the value of the dispersion slope, thedispersion value or the effective area.

European patent application No. 0 775 924 relates to a single modeoptical fibre having a three-segment index profile. Although the zerodispersion wavelength is in the 1520–1600 nm range, the total dispersionslope may be ≦0.095 ps/μm².km. No details are provided with regard tothe slope required for obtaining a satisfactory equilibrium ofcharacteristics as regards the dispersion slope, macro bending andeffective area.

In the case of very long distances (distances of 1000 km and longer)optical fibres for high transmission rates are optimised for use inwavelength range around 1550 nm, in which wavelength range the opticalattenuation may be considered low (about 0.2 dB/km). The NZDF (“non-zerodispersion fibres”) also have shifted dispersion, as a result of whichthe dispersion at a wavelength of 1550 nm is lower than that of astandard single mode fibre. The dispersion value deviates sufficientlyfrom zero, however, for minimising the effect of non-linearcharacteristics that may have a seriously adverse effect on theallowable maximum transmission capacity.

Because of the relatively short distances in the aforesaid networks,less strict requirements are made of the optical attenuation, as aresult of which also the wavelength range around 1300 nm, which has acharacteristic attenuation of about 0.3 dB/km, is in principle suitablefor such applications. As a result of the optimisation thereof in the1550 nm wavelength range, the fibres used for long-distance transmissionare less suitable for use in the wavelength range around 1300 nm,however.

It is an object of the present invention to provide a single modeoptical fibre suitable for multichannel transmission in the wavelengthrange around 1550 nm, viz. 1440 nm−1625 nm, and the wavelength rangearound 1300 nm, viz. 1250 nm−1360 nm, using high transmission rates.

Another object of the present invention is to provide a single modeoptical fibre in which the profile of the optical fibre is designed toprevent stress variations that may lead to undesirable characteristicsof the optical fibre.

The single mode optical fibre as referred to in the introduction isaccording to the present invention characterized in that the refractiveindex value nil is substantially constant in the first central regionhaving radius r1, and in the second ring having radius r2 the refractiveindex value decreases in radial direction from n1 to n2 over a distancer1−r2, with the decrease, which is substantially linear, taking place inaccordance with the following equation:slope=(D1−D2)/(r2−r1), where0,11<slope<0,22, and

${D_{i} = {{{\frac{n_{i}^{2} - n_{Cl}^{2}}{2 \cdot n_{i}^{2}} \cdot 100}\%\mspace{14mu}{and}\mspace{14mu} n_{i}} > n_{Cl}}},$

D_(i)=refractive index contrast for position i,

n_(i)=refractive index of position i,

n_(C1)=refractive index of the outer fibre cladding.

In a special embodiment the central region, which has a substantiallyconstant refractive index, preferably has a radius r1 having a maximumvalue of 0.25 μm. If the radius r1 is larger than the aforesaid value,it is not possible to obtain a fibre having a sufficiently largeeffective area and a sufficiently low dispersion slope.

The present invention furthermore relates to a single mode optical fibrecomprising a first central region having a radius r1 and a maximumrefractive index value nil and at least one second ring surrounding saidfirst central region, which second ring has a radius r2 and a minimumrefractive index value n2, where n2<n1, which fibre is characterized inthat the refractive index value is substantially constant in the secondring and decreases in radial direction over a distance r1 from n1 to n2from the central axis of symmetry in the first central region, with thedecrease, which is substantially linear, taking place in accordance withthe following equation:slope=(D1−D2)/(r1), where0,11<slope<0,22, and

${D_{i} = {{{\frac{n_{i}^{2} - n_{Cl}^{2}}{2 \cdot n_{i}^{2}} \cdot 100}\%\mspace{14mu}{and}\mspace{14mu} n_{i}} > n_{Cl}}},$and n_(i)>n_(C1),

wherein the meaning of D_(i), n_(i) and n_(C1) is as indicated above.

The present invention furthermore relates to a single mode optical fibrecomprising a first central region having a radius r1 and a maximumrefractive index value n1 and at least one second ring surrounding saidfirst central region, which second ring has a radius r2 and a minimumrefractive index value n2, where n2<n1, which single mode fibre ischaracterized in that the refractive index value is substantiallyconstant in the second ring and decreases in radial direction over adistance r1 from n1 to n1″ from the central axis of symmetry in thefirst central region, with the decrease, which is substantially linear,taking place in accordance with the following equation:slope=(D1−D1″)/(r1), where0,11<slope<0,22, and

${D_{i} = {{{\frac{n_{i}^{2} - n_{Cl}^{2}}{2 \cdot n_{i}^{2}} \cdot 100}\%\mspace{14mu}{and}\mspace{14mu} n_{i}} > n_{Cl}}},$n_(i)>n_(C1) andn1>n1″>n2,

wherein the meaning of D_(i), n_(i) and n_(C1) is as indicated above.

The present inventors have accomplished their invention on basis of thisfinding, with the profile of the optical fibre being so designed as toprevent undesirable stress variations. Undesirable characteristics, suchas an increased PMD (or sensitivity to the hydrogen-induced attenuationlosses) are thus reduced to a minimum.

In specific embodiments, the second ring having radius r2 is preferablysurrounded by a third ring having a radius r3 and a refractive indexvalue n3, wherein n3<n2 and r3>r2. In addition, the third ring havingradius r3 may be surrounded by a fourth ring having a radius r4 and arefractive index value n4, wherein n4<n3 and r4>r3.

Preferably, the dispersion value of the present optical fibre is 8ps/nm.km or higher at 1550 nm, whereas the dispersion value at 1300 nmmust be −8 ps/nm.km or lower. When such dispersion values are used, itis possible to use several channels, viz. signals having differentwavelengths, simultaneously in the two aforesaid wavelength ranges athigh transmission rates of 10 Gbit/s or higher, without the non-linearcharacteristics having a limiting effect in this regard.

The present inventors have furthermore discovered that the slope in therefractive index profile is an important design parameter for achievingthe right combination of characteristics of the final optical fibre.Consequently, said slope preferably ranges from 0.11 to 0.22, inparticular from 0.13 to 0.19. If a slope having a value higher than theabove range of values is used, the macro bending losses and thedispersion slope will be too large, which is undesirable in practice.If, on the other hand, a slope having a value lower than the above rangeof values is used, the effective area will be too small, which is alsoundesirable in practice.

In order to enable the simultaneous transmission of a large number ofsignals at different wavelengths without significantly limiting thepower density, the present optical fibre preferably has an effectivearea of 60 μm² or more at a wavelength in the 1550 nm range.

Furthermore preferably, the dispersion slope for the present opticalfibre ranges from 0.07 to 0.095 ps/nm.km² at 1550 nm.

In order to ensure that the present optical fibre can be considered tobe a single mode optical fibre over a maximum wavelength range, thecut-off wavelength is preferably lower than 1200 nm, measured for anoptical fibre having a length of 2 m.

In addition to having the features of the aforesaid preferredembodiments, the optical fibre should exhibit low losses caused bybending of the optical fibre. The fact is that many connections are madein the aforesaid networks consisting of optical fibres, for whichconnections loops are usually laid in the fibre at the locations of saidconnections. Thus it is desirable to limit the attenuation losses causedby such loops as much as possible, which preferably implies that themacro bending losses, measured at a wavelength of 1625 nm and 100windings having a bending diameter of 60 nm, are preferably lower than0.05 dB.

The present invention furthermore relates to an optical communicationsystem for multi-channel signal transmission, which system ischaracterized in that the present fibre is used as a transmission mediumfor several channels in the wavelength range of either 1550 nm or 1300nm.

The present invention will be explained in more detail hereinafter bymeans of embodiments, in which connection it should be noted, however,that the present invention is by no means limited to such embodiments.In the appended FIGS. 1–3, the refractive index profiles of a number ofoptical fibres according to a special embodiment of the invention areschematically shown as a function of the radius.

FIG. 1 shows a possible refractive index profile of a fibre according tothe present invention. The position indicated at reference number 1 onsaid profile is a maximum refractive index substantially on the axis ofsymmetry, which position has a refractive index value n1 and arefractive index difference D1. The position indicated at referencenumber 2 is spaced from the axis of symmetry by a distance r1 and has adistinctly lower refractive index value n2 and refractive indexdifference D2 than the position indicated at reference number 1. Therefractive index value decreases in practically linear, monotonousfashion from position 1 to position 2, with the slope h being inaccordance with the following equation: h=(D1−D2)/(r1). The positionindicated at 3 is spaced from the axis of symmetry by a distance r2.Finally, numeral 4 indicates a position which is spaced from the axis ofsymmetry by substantially the same distance as position 3, whichposition 4 shows the refractive index value of the cladding, whichcladding extends from the axis of symmetry starting on a distance r2 andhas a refractive index value lower than that of position 3.

FIG. 2 shows a refractive index profile of a fibre according to thepresent invention. A circularly symmetric profile having a distance r1is built up around a central axis of symmetry, in which the positionindicated at reference number 1 of said profile requires a refractiveindex having a constant refractive index value n1. The positionindicated at reference number 2 is spaced from the axis of symmetry by adistance r2 and has a distinctly lower refractive index value n2 andrefractive index difference D2 than the position indicated at referencenumber 1. The refractive index value decreases in practically linear,monotonous fashion from position 1 to position 2, where the slopeh=(D1−D2)/(r1). The position indicated at reference number 4 indicatesthe refractive index value of the cladding, which cladding extends fromthe axis of symmetry starting on a distance r2. In FIG. 2, n1>n2>nc1.

FIG. 3 shows a refractive index profile of a fibre according to thepresent invention. The position indicated at 1 on said profile is amaximum refractive index substantially on the axis of symmetry, whichposition has a refractive index value n1 and a refractive indexdifference D1. The position indicated at 1″ is spaced from the axis ofsymmetry by a distance r1 and has a distinctly lower refractive indexvalue n1″ than the position indicated at 1. The refractive index valuedecreases in practically linear, monotonous fashion from position 1 toposition 1″ already. The position indicated at 2 is also spaced from theaxis of symmetry by a distance r2 and has a refractive index value n2and a refractive index difference D2. Finally, reference number 4indicates a position which is spaced from the axis of symmetry bysubstantially the same distance as position 3, from which position 4 thecladding begins, which cladding has a refractive index value lower thanthat of position 3.

As already explained before, the slope h is an important designparameter for obtaining the right combination of characteristics of theoptical fibre. Said slope h can be influenced by, for example, adaptingthe refractive index difference of position 2. The slope can beincreased by causing the index value of position 2, in particular inFIGS. 1 and 2, to decrease. In addition to that, adaptation of the slopecan also take place by changing the radial position of position 1″ (seeFIG. 3) and position 2 (see FIGS. 1 and 2). As already said before, thepresent inventors have discovered that the slope preferably ranges from0.11 to 0.22, more in particular from 0.13 to 0.19. In the case of aslope having a value higher than the aforesaid range of values the macrobending losses will increase undesirably, whilst the result of a slopehaving a value lower than the aforesaid range of values will be that thedesired effective area will be too small.

The influence of the slope in the refractive index profile of theprofiles presented herein on a number of parameters, viz. the dispersionslope, which is preferably lower, than 0.095 ps/nm.km², the macrobending losses, which are preferably lower than 0.05 dB (measured at awavelength of 1625 nm, windings having a bending diameter of 60 mm) andan effective area, which is preferably larger than 60 μm², has beeninvestigated for optical fibres that meet the dispersion requirement of≧8 ps/nm.km at 1550 nm and ≦8 ps/nm.km at 1330 nm and the cut-offwavelength requirement of <1200 m, measured on an optical fibre having alength of 2 m. The results are presented in the table below.

Slope Dispersion slope Macro bending Effective area 0,10 ++ ++ − 0,12 +++ +/− 0,14 + + + 0,16 + + + 0,18 + + + 0,20 +/− + + 0,22 +/− +/− ++0,24 +/− −− ++

The above table clearly shows that a satisfactory equilibrium ofcharacteristics is obtained with a the slope in the range0,11<slope<0,22.

1. Single mode optical fibre comprising a first central region having aradius r1, a maximum refractive index value n1 and at least one secondring surrounding said first central region, which second ring has aradius r2 and a minimum refractive index value n2, wherein n2<n1,characterized in that the refractive index value n1 is substantiallyconstant in the first central region having radius r1, and in the secondring having radius r2 the refractive index value decreases in radialdirection from n1 to n2 over a distance r1−r2, with the decrease, whichis substantially linear, taking place in accordance with the followingequation:slope=(D1−D2)/(r2−r1), where0.11<slope<0.22, and${D_{i} = {{{\frac{n_{i}^{2} - n_{Cl}^{2}}{2 \cdot n_{i}^{2}} \cdot 100}\%\mspace{14mu}{and}\mspace{14mu} n_{i}} > n_{Cl}}},$D_(i)=refractive index contrast for position i, n_(i)=refractive indexof position i, n_(C1)=refractive index of an outer fibre cladding,wherein a dispersion slope ranges from 0.07 to 0.095 ps/km² at 1550 nm.2. Optical fibre according to claim 1, wherein r1<0.25 μm.
 3. Opticalfibre according to claim 1, characterized in that the second ring havingradius r2 is surrounded by a third ring having a radius r3 and arefractive index value n3, wherein n3<n2 and r3>r2.
 4. Optical fibreaccording to claim 1, characterized in that the third ring having radiusr3 is surrounded by a fourth ring having a radius r4 and a refractiveindex value n4, wherein n4<n3 and r4>r3.
 5. Optical fibre according toclaim 1, wherein 0.13<slope<0.19.
 6. Optical fibre according to claim 1,characterized in that the dispersion value is 8 ps/km or higher at 1550nm.
 7. Optical fibre according to claim 1, characterized in that thedispersion value is −8 ps/km or lower at 1300 nm.
 8. Optical fibreaccording to claim 1, characterized in that the effective area is 60 μm²or more at 1550 nm.
 9. Optical fibre according claim 1, characterized inthat the cut-off wavelength is lower than 1200 nm, measured for a fibrehaving a length of 2 m.